Cutting tools, including cemented carbide cutting tools, have been used in both coated and uncoated conditions for machining various metals and alloys. In order to increase cutting tool wear resistance and lifetime, one or more layers of refractory materials have been applied to cutting tool surfaces. TiC, TiCN, TiN and Al2O3, for example, have been applied to cemented carbide substrates by chemical vapor deposition (CVD). Moreover, in recognizing some shortcomings of CVD coatings related to specific cutting applications, cutting tool manufacturers have also provided refractory coatings applied by PVD. TiN coatings applied by PVD, for example, have been generally accepted in the cutting tool community.
One disadvantage of TiN coatings is the susceptibility to oxidation at relatively low temperatures. TiN coatings, for example, demonstrate oxidation beginning at about 550° C. As a result, aluminum has been added to TiN coatings in efforts to increase the oxidation resistance. Silicon has additionally been added to TiN coatings to increase oxidation resistance. However, the addition of silicon to TiN and/or TiAlN coatings can induce significant stresses in the coatings thereby inducing premature coating failure by delamination from the cutting tool surface.
U.S. Pat. No. 6,586,122 addresses methods of incorporating silicon into TiN and TiAlN coatings to increase oxidation resistance while reducing residual compressive stresses of the coatings. The synthetic methods disclosed in U.S. Pat. No. 6,586,122 induce a phase separated coating wherein a high silicon concentration Si3N4 nanophase is dispersed throughout a low silicon concentration TiSiN matrix phase. Dispersing a high silicon concentration Si3N4 nanophase throughout a low silicon concentration matrix phase can reduce lattice strain induced by the substitution of Ti with Si in a TiN or TiAlSiN coating.
The synthetic methods required to induce such a phase separated coating, nevertheless, are unconventional and require alteration of traditional PVD methods and/or equipment, thereby potentially limiting widespread application of the methods and increasing coated tool production costs.